The present invention is related to lamps, particularly sealed beam lamp units, having two-piece envelopes comprising a reflector and lens assembled by adhesive means. The reflector has an internal reflective coating for reflecting and directing light, originating from a light source located within the envelope, towards a cooperating lens through which the light is transmitted. Such sealed beam lamps have particular utility and are commonly used as headlights for motor vehicles.
Lamp units, such as headlights, reently have been introduced with lenses and reflectors having rectangular shaped peripheries, supplanting the more familiar circular units. Production of assembled rectangular glass reflectors and lenses, however, can present numerous problems. For example, stresses created in the glass lenses and reflectors during assembly by fusion sealing can cause cracking thereof. Such stresses can be significantly reduced by using an adhesive, rather than fusion, to seal the glass reflector and lens together. However, the ambient temperature, particularly at cold temperatures, can produce additional stresses due to the differences in the coefficients of expansion between glass and adhesive resulting in cracks, especially about the reflector corners. Thus, the inherent problem remains of thermally induced stresses experienced when dissimilar materials, such as glass and adhesive, are joined.
Additionally, adhesively bonded lamps, which are generally made from moldable materials, and especially from glass, are very susceptible to flaws. More particularly, in the manufacture of the lamp unit, moldable materials such as, but not limited to, glass typically have flaws on the surfaces thereof. If the adhesive, which seals the reflector and lens together, is in contact with these flaws, the adhesive at cold temperatures will contact at a different rate than the glass and thereby place stress on one of the weaker portions of the glassware, namely the flaws. Thus the inherent problem of thermally induced stresses experienced when dissimilar materials are joined is aggravated by adhesive contact with flawed areas.
Furthermore, if any portion of the adhesive flows onto the light-transmitting area of the lens or light-reflecting area of the reflector, or beyond the lamp unit periphery during assembly of the unit, undesirable and unacceptable lamp unit optical performance and/or peripheral dimensions can result. Still further, due to the prior art sealing surfaces shapes, more adhesive than desired is required.
Prior art lamp units also require a relatively high adhesive flexibility to reduce tearing or spalling of the adhesive due to the different coefficients of expansion for glass and adhesive. Such high adhesive flexibility is achieved through the addition of flexibilizers in the adhesive. Flexibilizers, however, undesirably increase the permeability of the adhesive resulting in moisture penetration leading to filament and lamp degradation.